With an alloy of the same overall composition as M3HA, but having a conventional Sr level of less than 0.1%, primary Si particles form on or in the vicinity of the mould wall as the melt is poured. Subsequent incoming melt washes the particles into the body of the melt. If cooling of the melt is sufficiently slow, the primary Si particles grow and can become large in size. If, on the other hand, solidification of the melt is relatively rapid, growth of the particles is substantially avoided. In either case, the relatively low density primary Si particles tend to float, giving rise to segregation, with the adverse consequences of this being more severe with large particles.
In the case of M3HA alloy, Sr combines with Al and Si in the melt to form intermetallic particles of an Al-Si-Sr phase. It is these particles rather than primary Si, which form on or in the vicinity of the mould wall at the start of melt pouring and are then swept into the body of the melt. The formation of the Al-Si-Sr phase changes the conditions at the mould walls in that it allows the mould to heat up before the formation temperature for primary Si is reached. As a result, Si formation at the mould walls is suppressed. In the absence of Ti in M3HA, the Sr intermetallic particles form predominantly as undesirable platelets. However when Ti is present, these Al-Si-Sr intermetallic particles form as equi-axed particles, except when the solidification rate is very high, in which case the particles can form as platelets.